Learn English – How to pronounce the “v” sound

The general rule of thumb is to read it as you would any other word, i.e., follow normal rules of pronunciation. It'll work most of the time, and when you're off, you'll be told the correct, "unorthodox" pronunciation.

I can't speak for all Americans; however, in my elementary school we did read texts aloud, but rarely. And I don't remember specifically being taught how to pronounce unfamiliar proper nouns.

I say don't worry about it too much--if it's unfamiliar to you, it's likely to be unfamiliar to many others. In this case, no one will fault you for not knowing how to pronounce the word; as you said, it can't be looked up in a dictionary.

In standard English, 'th' is always a dental fricative, a non-stopping, continuous movement of air between the tip of the tongue right and the bottom of the top teeth. It is almost exactly the lisp sound.

It is a rare sound in the world's languages, so most language leaners of English have trouble pronouncing it and attempt to approximate it by the fricatives s/z or the stop t/d.

There are some non-standard varieties of English that use t/d naturally instead of the fricative, like for example Sylvester Stallone in Rocky, or any number of street-wise New Yorkers. So don't emulate this unless you want to sound very urban (and a bit out-of-date)

As with most phonetic things, learning it comes with practice and listening and more practice. For 'th' in particular, a little bit of your tongue sticks past the top teeth, so may feel weird (or even rude depending on your culture).